Method for locating impedance discountinuities in a wave propagating medium

ABSTRACT

A method for digitally processing seismic data, or other similar data derived for propagating wave energy to produce a data image comprised of a plurality of coordinate points is described. Each coordinate point has a value which is a combination of the amplitude values taken from a plurality of primary seismic traces at the travel times on the respective primary traces required for energy to travel from the respective source points to the coordinate point and return to the respective receiving points of the seismic traces. More particularly, the combination is accomplished in such a way as to increase the signal-to-noise ratio by selected data to be combined at the respective coordinate points either on a predetermined basis or automatically by an analysis of the data.

(mite States Patent Backus et al.

[54] METHOD FOR LOCATING 3,539,984 11/1970 Schneider ..340/ 15.5 CC

IMPEDANCE DISCOUNTINUITIES IN A WAVE PROPAGATING MEDIUM Primary Examiner-Benjamin A. Borchelt Assistant Examiner-H. A. Birmiel [72] Inventors 31 b ti f Attorney-Samuel M. Mims, Jr., James 0. Dixon, Annel 0 O D drew M. Hassell, Harold Levine, Rene E. Grossman, [73] Assignee: Texas Instrument Incorporated, Dal- Melvin ShaIP and Richards, Harris & Hubbard 57 ABSTRACT 22 Filed: July 6, 1970 l 1 A method for digitally processing seismic data, or I PP 52,512 other similar data derived for propagating wave energy to prmlucc a data image comprised of a plurality of 57 y CL 340/155 s coordinate points is described. Each coordinate point 340/l5 5MC 340l5CC has a value which is a combination of the amplitude 51 1 1m. CI. ..G01v 1/28 values take" a plurality of Primary Seismic 58 Field of Search ..340/15.5 MC 15.5 DP at the travel times the rPective Primary "aces required for energy to travel from the respective source points to the coordinate point and return to the [56] References Cited respective receiving points of the seismic traces. More particularly, the combination is accomplished in such UNITED STATES PATENTS a way as to increase the signal-to-noise ratio by selected data tobe combined at the respective coor- 5 1 l l 1/1967 Rockwell "340/ 15 CD dinate points either on a predetermined basis or auto- 3,504, 94 2/[971 FY3816! et al ..340/15.5 MC matically y an analysis of the data. 3,529,282 9/1970 Brown ..340/15.5 MC 3,105,568 10/1963 Jolly ..340/l5.5 CC 11 Claims, 29 Drawing Figures SPREAD TRACE a SPREAD TRACE 20b SPREAD TRACE j (FIG 4) (FIG. 4) (FIG. 4)

2Iu 21b 2Ic SI R200 -S| R20" SI R c| -S R o 5 R4 o S R4 o CORRECT FOR 66 CORRECT FOR 64 CORRECT FOR MOVEOUT(FIG.5) 2 MOVEOUT (F165) 22b MOVEOUT (F165) "2 23a 23b 23c s R2 b S R b 5 R b -S R b s R4 b -S R b SECTOR SELECTION SECTOR SELECTION F\ SECTOR SELECTION (FIG. s) 240 (FIG. 8) 241: (FIG. 8)

25a 25b 25c 5 R b -S R h s R ti -S R b s R b -S R b 80 827 6/20 6/00 62b b 62C 60c j j CORRELATION CORRELATION CORRELATION CORRELATION CORRELATION CORRELATION GAIN FOR GAIN FOR GAIN FOR GAIN FOR GAIN FOR GAIN FOR DIFFRACTORS REFLECTORS DIFFRACTORS REFLECTORS DIFFRACTORS REFLECTORS (FIG IO) (FIG 9) (FIG. IO) (FIG. 9) (FIGIO) (FIG. 9)

T 29a I 270 I 29b I 27!: T 25 27c Is R d -S a n l ls R C S R c Is R d -S R h [5 R c s R c l FR4 IO-SI R d I fs a c -s R cd I i i i i i s COMPOSITE IMAGE K Patented Aug. 29, 1972 16 Sheets-Sheet 1 FIG. I

Patented Aug. 29, 1972 16 Sheets-Sheet 5 lllllllllllllllllllllll n 0 L l J! O fl v l l l i I l I I l l I llvll 9 I l l l I l l II 9 l l l l l I l l llu-Il G7 8 l l I I I I I I I I l II 9 I I |l+|| 0 0 2 R 6 I l I I II 2 9 I I I I I I I I 1 i I 4 0 5 9 I l l l l l l l I l I I I l I l l lI-Ill 3 l l l l I I I l l II 9 I I T o 2 I I 1 I l l I I l ll FIG. 4

Patented Aug. 29, 1972 16 Sheets-Sheet 4.

FIG. 7

Patented Aug. 29, 1972 16 Sheets-Sheet 5 WILL [AM A. SCHNEIDER Patented Aug. 29, 1972 16 Sheets-Sheet 6 S R b S R b FIG. 9

m. m mw c EKS c WAA. M A on NJ MW FIG. 9a

Patented Aug. 29, 1972 16 Sheets-Sheet 7 INPUT TRACE 5 IOOb J SPREAD TRACE (FIG. 4)

CORRECT FOR MOVEOUT (F|G.5)

INPUT TRACE P6 SPREAD TRACE (FIG. 4)

I CORRECT FOR MOVEOUT (FIGS) I I l I IOBb l I COLLATE TRACES AT STATIONS TRACES FOR STATION 94 (FIG.I2)

ISECTOR SELECTIONI 'L CALCULATE CORRELATION GAIN COORDINATE VALUES FOR STATION 94 OF DATA IMAGE TRACES FOR STATION 95 (FIG.I2)

SECTOR SELECTION] "1 CALCULATE CORRELATION GAIN COMBINE TRACES w I COMBINE TRACES I L FOR STATION 95 OF DATA IMAGE COORDINATE VALUES FIG. 1/

I TRACES 6 (FIG. I2)

STATION 9 I I SECTOR SELECTION CALCULATE CORRELATION GAIN I I COMBINE TRACES I L COORDINATEVALUES FOR STATION 96 OF DATA IMAGE INVENTORSI MILO M. BACKUS WILLIAM A. SCHNEIDER 16 Sheets-Sheet 8 STATIONS FIG. 12

INVENTORS'.

MILO M BACKUS WILL/AM A. SCHNEIDER S F" b -S R b FIG. /0

Patented Aug. 29, 1972 16 Sheets-Sheet 10 FIG /4 FIG. [5

INVENTORS:

MILO M. BACKUS WILLIAM A. SCHNEIDER Patented Aug. 29, 1972 16 Sheets-Sheet 11 l 2 3 4 5 P6 7 a "9 FIG. 16

. INVENTORS'. MILO M. BACKUS WILLIAM A. SCHNEIDER FIG. /7

Patented Aug. 29, 1972 16 Sheets-Sheet 12 N MODPDQSZ N mODFIASE b 2 M b I 5g b O 0 FIG. 2/

STATION 95 GROUP 8 GROUP C GROUP A Patented Aug. 29, 1972 16 Sheets-Sheet 15 INVENTORS:

MILO M. BACKUS WILLIAM A. SCHNEIDER MN UK Patented Aug. 29, 1972 16 Sheets-Sheet 14.

228i 229i 230! [X v'r'w 228h 229b 230b In] WW SH +o-H-i 22m V -229e -230e I }.-.:r'.:::::::'-:{:'::'1':{

228d 229d 230d 11 III 228b 229b 230b H M FIG. 25

TRACES g b AT STATION 95 STACK ANGLE SECTIONS 1- IX [223D L ANGLE TRACESI-IX 1 y f 224D COMPUTE POWER CONTENT AND SCALERS APPLY SCALERS TO TRACES I-IX y r 120D STACK TRACES TO PRODUCE COORDINATE POINT VALUES AT STATION 95 FIG. 24

INVENTORS'.

M/LO M. BACKUS WILLIAM A. SCHNEIDER Patented Aug. 29, 1972 16 Sheets-Sheet 15 FIG. 26

FIG; 2a

INVENTORS'.

MILO M BACK US WILL/AM A. SCHNEIDER Patented Aug. 29, 1972 16 Sheets-Shet l6 FIG. 27

INVENTORS:

MILO M BACKUS WILLIAM A. SCHNEIDER METHOD FOR LOCATING IMPEDANCE DISCOUNTINUITIES IN A WAVE PROPAGATING MEDIUM This invention relates generally to methods and apparatus for processing data representative of wave energy induced at a source point in a wave propagating medium and detected at a receiving point in such a manner as to produce a data image of discontinuities within the medium. The invention is specifically related to methods for determining the location of both diffraction and reflection type acoustic impedance discontinuities of a planet such as Earth.

Extensive research efiorts have been expended in recent decades to perfect the art of Seismology. Recent events indicate that one of the more significant developments in this art is the method for processing seismic data described and claimed in US. Pat. No. 3,353,151, entitled Common Tangent Stacking Method And System For Locating Sonic Energy Velocity Discontinuities, issued to D. W. Rockwell on Nov. 14, 1967, and assigned to the assignee of this invention.

The Rockwell patent discloses a method for processing seismic data wherein each seismic response record, commonly referred to as a trace, is plotted by either analog or digital apparatus, as a scaled wavefront pattern. Each coordinate point of the scaled wavefront pattern has the amplitude value at the travel time on the trace required for the energy to travel from the source point to the coordinate position and then back to the receiving point. The amplitude values of the scaled wavefront patterns derived from a plurality of traces are then combined at corresponding coordinate points which results in a composite image of the acoustic impedance discontinuities of the subsurface. Stated another way, imagine the volume being explored as a three-dimensional grid. For each source-receiver trace recorded, the travel time from the source to a grid point in the volume and back to the receiver is calculated. The amplitude value of the recorded trace, observed at the calculated time, is added to an accumulating sum associated with the grid point. The resulting set of values obtained over a region of the subsurface grid, as a result of accumulating data from many sourcereceiver traces, will then show large values at the location of subsurface scatters, such as fault planes, dome boundaries, reefs, unconformities, pinchouts, cavities, rough boundaries, etc., as well as reflecting interfaces. The grid of numbers may be displayed in a number of conventional ways. The composite image so produced has recently been recognized as a form of acoustic hologram, and the method a form of acoustic holography.

There are many advantages of the Rockwell method when compared to previous data processing methods. Perhaps the most significant advantage is that each incremental time interval is plotted in the true migrated mode with respect to either two or three-dimensional space. The method tends to automatically enhance the signal-to-noise ratio. No special constraints in the geometry of the source and receiver are required. The process is applicable to either reflection, diffraction or refraction energy, and in particular provides, for the first time, a practical method for locating energy diffractors or scatterers.

The present invention is concerned with an improvement in the method described and claimed in the above referenced Rockwell patent, and more particularly relates to a method for processing the data so as to further enhance the signal-to-noise ratio and to thus better identify and locate both reflectors and diffractors.

In accordance with this invention, each input trace, derived either from a single source-receiver pair or by stacking the traces from a plurality of source-receiver pairs, is spread to a number of horizontally spaced stations. Then each spread trace is corrected for moveout to the respective station based on the locations of the source and receiver for the input trace relative to the station so that each of the spread traces is corrected to verticality. Then the vertically corrected traces at each interval are combined in a predetermined manner to produce a composite data image. The data image is a coordinate grid. The value at each coordinate point of the grid is a positive or negative value, and the image may be further processed for display in any conventional manner.

More specifically, the invention contemplates enhancement of the composite data image by controlling, either on a predetermined or an automatic basis, the input traces which are to contribute data to certain areas of the composite image. In accordance with a specific aspect of the invention, the detection of specular reflectors is enhanced by selecting only a portion of the vertically corrected traces at the respective stations for combination to produce the composite data image based on a predetermined range of dip angles for the reflectors. This selection may include a scaling or weighting of the values, and may change with depth.

In accordance with another aspect of the invention, specular reflectors can be enhanced by automatically increasing the amplitude values of certain of the vertically corrected traces in accordance with the degree of correspondence between the amplitude values of a particular trace and the amplitude values of one or more collateral traces at the travel times on the collateral traces where energy reflected from the said discontinuity would be found if a discontinuity existed.

In accordance with still another aspect of the invention, the image of scattering or diffracting discontinuities can be enhanced by automatically increasing the amplitude values of the vertically corrected traces at each station in accordance with the degree of correspondence between said amplitude values and the amplitude values on one or more collateral traces at the travel times where energy would be returned from a diffracting discontinuity located at the point represented by said amplitude values.

The novel features believed characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and ad vantages thereof, may best be understood by reference to the following detailed description of illustrative embodiments, when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of a two-dimensional composite data image in accordance with the present invention;

FIG. 2 is a schematic diagram illustrating a conventional method of collecting multicoverage seismic data using common depth point geometry which can also be advantageously processed by the method of the present invention;

FIG. 3 is a block diagram which illustrates one embodiment of the method of the present invention;

FIG. 4 is a schematic diagram which illustrates one step of the method represented in FIG. 3;

FIG. 5 is a schematic diagram which illustrates another step in the method represented in FIG. 3;

FIG. 6 is a schematic diagram which also illustrates the method step illustrated in FIG. 5;

FIG. 7 is a table setting forth the traces which are assigned to the respective stations in accordance with the method of FIG. 3;

FIG. 8 is a schematic diagram which serves to illustrate the criteria for the step of sector selecting in the method of FIG. 3;

FIG. 9 is a schematic diagram illustrating the method of automatic sector selection by correlation gain in the method of FIG. 3;

FIG. 9a is a schematic diagram illustrating a method for determining correlation gain in the sector selecting step of FIG. 9;

FIG. 10 is a schematic diagram illustrating sector selection by automatic correlation gain for diffractors in the method of FIG. 3;

FIG. 11 is a block diagram of another embodiment of the method of the present invention;

FIG. 12 is a table illustrating the manner in which traces are collated at the respective stations in the method of FIG. ll;

FIG. 13 is a schematic diagram which illustrates specific aspects of the method of FIG. 11;

FIGS. 14-17 are schematic diagrams which illustrate how the data displayed in FIG. 13 is interpreted;

FIGS. 18-21 are schematic diagrams illustrating amplitude profiles of the data displayed in FIG. 13 at depths Z Z respectively;

FIG. 22 is a schematic diagram which illustrates advantages of the method of the present invention;

FIG. 23 is a schematic diagram which illustrates another embodiment of the method of the present invention for automatic sector control;

FIG. 24 is a block diagram of the embodiment of the invention illustrated in FIG. 23;

FIG. 25 is a schematic diagram which illustrates one aspect of the embodiment of FIG. 23;

FIG. 26 is a schematic drawing which illustrates application of the method of the present invention to produce a three-dimensional data image;

FIG. 27 is another schematic drawing which illustrates the application of the present invention to produce a three-dimensional image from a plurality of sets of common depth point data; and

FIG. 28 is a schematic diagram which illustrates one step of the method as applied to the production of a three-dimensional data image.

Referring now to the drawings, FIG. 1 illustrates schematically the type of data image which is to be produced in accordance with the present invention. A datum line D represents a horizontal line which may approximate the surface-air interface of the Earth. The datum line D is divided at equal horizontal intervals to provide stations 86-100, which, of course, may be only one portion of the total image. The data image is comprised of a plurality of coordinate points 10 arranged at equal vertical intervals under each of the stations 86-100 on the datum line D. The stations on the datum line D are typically to feet apart, while the vertical intervals between the coordinate points 10 may be expressed either in equal time increments, or equal depth increments. In either case, the increments will typically represent from 25 to 50 feet. The vertical intervals will hereafter be expressed in depth for convenience unless otherwise stated, although it will be understood that the method may be practiced using either the time or depth domain as in more conventional common depth point stacking.

Each of the coordinate points 10 is comprised of a numerical value, either positive or negative, which is usually expressed in digital machine language. The value at each of the coordinate points of the data image is derived by combining a plurality of amplitude values taken from a plurality of input traces derived from source-receiver pairs as will hereafter be described in detail. The data image shown in FIG. 1 is the type described in the above referenced Rockwell patent. This invention is concerned with an improved method for processing the data to produce a data image having enhanced images.

A conventional method for collecting seismic data, commonly referred to as the common depth point method is illustrated in FIG. 2. In FIG. 2, lines 10a-l0c represent the surface of either land or water, which overlies a subsurface region in which it is desired to map the acoustic impedance discontinuities.

Assume that seismic energy is first induced at a source point S commonly referred to as a shot point, using any conventional means, such as an explosive charge, gas exploder, hydraulic thumper, weight drop or the like. Assume also that shot point S is horizontally located at station 90 on datum line D, although the shot point S may be above or below the elevation of the datum line. An array of receivers is disposed on the surface 10a to one side of shot point 8, with the receivers located at stations 92, 94, 96, etc. Twentyfour receivers are typically used although only receivers R,R are illustrated in FIG. 2. It should be noted that the receivers are uniformly spaced, and that shot point S is typically spaced from receiver R by a distance equal to the spacing between the receivers. The acoustic return detected at receivers R -R together with that detected at the other 19 receivers of the spread is typically recorded on a single 24 trace seismogram. These traces may be individually identified by a combination of the shot point designation and the receiver designation, such as traces 8 R,- S R respectively. The values of each of the traces of the seismogram are preferably digital values representing both the amplitude and the phase of the detected acoustic energy. The digital values are typically produced by sampling the detected analog signal at time intervals of one, two or four milliseconds and are recorded on a single multiplexed channel.

After the first seismogram is produced from energy induced at shot point 8,, the source is indexed to station 88, as represented by shot point S and each of the receivers R,-R is also indexed two stations, as illustrated on line 10b, and the shooting process repeated. This produces a second seismogram containing twentyfour traces, which traces can be identified as traces S R,-A:S R respectively. The shooting procedure is then repeated with shot point S at station 86 and the receivers Il -R spread at the stations illustrated on line 100. The shooting procedure is repeated every two stations with the appropriate movement of the receiver spread until the entire area of interest is completed. It is important to understand that the relationship between the shot points, receivers and stations is not critical to the invention, but is a convenience for handling the data in an orderly fashion. In general, a wider spread of stations contributing to the image provides greater horizontal resolution, and a closer densityof stations provides greater vertical resolution in the composite data image.

Seismic data collected in this manner is referred to as common depth point data. Common depth point data is chosen to illustrate the method of the present invention because the method is ideally suited to process such data, and vast quantities of common depth point data heretofore collected can be advantageously processed in accordance with the method of the present invention to yield better information about the subsurface region than was heretofore possible. However, it is important to understand that the method of the present invention is not limited to common depth point data, but is applicable in its broader aspects to data representative of any wave energy induced at a source point and detected at a receiving point on or within a wave propagating medium such as the Earth, the human body, or any other solid liquid or gaseous body, or even outer space when using energy, such as electromagnetic wave energy, which will propagate through space.

Although the highly ordered nature of common depth point data simplifies data processing, it is to be understood that this invention requires no constraints upon the relative positions of the shot points and the receivers. The receivers need not be spaced at equal horizontal intervals, nor do the successive shot points need be spaced at equal horizontal intervals. Further neither the shot points nor the receivers need to be arrayed in a straight line, as is required for common depth point data. The shot points and the receivers may thus be located at any position, so long as the location of these positions are known. Further, a two-dimensional spread of shot points and receivers can be used to achieve a three-dimensional data image from which any desired two-dimensional section, vertical, horizontal, or at any angle, can be extracted for display, or which may be displayed using three-dimensional display techniques as stereo pairs, holograms, or more conventional three-dimensional drawings.

Each of the twenty-four traces S R S R on each of the seismograms produced from shots S -S,,, respectively, may be an input trace for the method illustrated in FIG. 3, and in all specific examples hereafter discussed, it is assumed that such is the case. However, it is to be understood that the input traces may be preprocessed in substantially any conventional manner prior to input to the method illustrated in FIG. 3, such as for example, to move the shot points and receivers to the datum line, correct for the overburden, or similar techniques. For best results, however, it is desirable to use original traces prior to common depth point stacking since such stacking distorts the data. However, because of the very large number of traces typically produced when collecting common depth point data, it may be desirable to select only a portion of the total numbered traces derived from difierent sourcereceiver pairs for processing in order to reduce the operating time of the digital computer. For some applications, common depth point stacked traces may be used as the input traces to the method, as hereafter described in connection with the embodiment of FIG. 11.

FIG. 3 is a block diagram illustrating the manner in which input traces S R S R and S,R,, for example, would be processed by a digital computer in accordance with one embodiment of the present invention. In general, all input traces are processed in parallel as. indicated by the identical vertical flow paths for the input traces S R S R and S R, shown in FIG. 3. For example, the input traces S 12 S R and S R are first spread as represented at blocks 20a, 20b and 200, respectively.

The spreading operation of block 20a, for example, is illustrated in detail in FIG. 4 and comprises merely reproducing the input trace S R to produce a plurality of identical traces S R a S R a,,. Since shot point S, was located at station and receiver R at station 94, the traces S R a -S,R a,, are spread consecutively from station 92, which is the center point between shot point S, and receiver R over the higher numbered stations. It will be noted that this occupies only a half space of the total sector from which the acoustic energy could have been returned to the receiver. Assuming that the acoustic velocity is the same on each side of the center point of the spread, traces S,R a S,R a,,, and all further traces derived therefrom, can be reproduced beginning at station 92 and proceeding to lower numbered stations. If the velocity is not the same when propagating in different directions from the center point, the processing of traces S R a -S,R a,, can be repeated using different velocity values and the traces designated by different characters, such as by -S,R a, through -S,R a,,. Since the traces S,R a -S,R a,, are merely duplications of the input trace S R the amplitude values at travel time T, on all traces would appear at the same apparent depth as illustrated in FIG. 4.

The outputs from steps 20a-20c are spreads of traces S1 R a S R a,,, S R a S R a and S R a S R a which are represented by blocks 2la-2lc, respectively, in FIG. 3, and which are assigned to the stations according to the table of FIG. 7. g

Next, each of the spreads of traces 21a-21c is corrected for moveout as represented by blocks 22a-22c, with spread 21a being illustrated in detail in FIGS. 5 and 6, to produce spreads 23a-23c, respectively, which includes traces S R b -S R b S R b -S R,,, and S R4b0S1R4b 11, respectively. Each trace of each spread 23a-23 is comprised of a series of amplitude values sampled at uniform depth intervals, I and are thus equivalent to a series of vertically spaced coordinate points. As mentioned previously, these uniform depth intervals can be uniform elapsed time intervals, rather than depth intervals. The depth intervals of the trace spreads 2311-230 are conveniently the same depth intervals as the coordinate points 10 of the composite data image illustrated in FIG. 1, although for some purposes 

1. The method for determining the location of impedance discontinuities in a wave propagating medium from a plurality of digital input records each representative of wave energy returning to a receiving point from wave energy induced at a source point which comprises producing, in an automatic data processing machine, a data image having a plurality of coordinate points defining at least two spacial dimensions, each coordinate point having a composite value produced by combining a plurality of weighted amplitude values, each weighted amplitude value being derived by picking the amplitude value on a different input trace at the time required for wave energy to travel from the source point of each of the respective input records to the coordinate point and return to the receiving point for the respective input record and multiplying each of the picked amplitude values by a weighting factor that is a function of the depth and the horizontal distance between the source points and receiver points of the input trace from which the amplitude value is picked and the coordinate point for which the value is to be derived, wherein the weighting factor at a given depth increases as the horizontal distance increases to enhance the image of diffractors.
 2. The method for determining the location of impedance discontinuities in a wave propagating medium from a plurality of digital input records each representative of wave energy returning to a receiving point from wave energy induced at a source point which comprises producing, in an automatic data processing machine, a data image having a plurality of coordinate points defining at least two spacial dimensions, each coordinate point having a composite value produced by combining a plurality of weighted amplitude values, eaCh weighted amplitude value being derived by picking the amplitude value on a different input trace at the time required for wave energy to travel from the source point of each of the respective input records to the coordinate point and return to the receiving point for the respective input record and multiplying each of the picked amplitude values by a weighting factor that is an inverse function of the degree of correspondence between preselected amplitude values, the degree of correspondence being indicative of spectral reflectors.
 3. The method of claim 2 wherein the coordinate points define three spacial dimensions.
 4. The method for determining the location of diffractive impedance discontinuities in a wave propagating medium from a plurality of digital input records each representative of wave energy returning to a receiving point from wave energy induced at a source point which comprises producing, in an automatic data processing machine, the data image having a plurality of coordinate points defining at least two spacial dimensions, each coordinate point being a composite value derived from a combination of amplitude values, each amplitude value being the amplitude value at the time on a different input trace required for energy to travel from the source of the respective input trace to the coordinate point to the receiving point for the respective record, the amplitude values for each coordinate value being selected substantially only from those input traces having the midpoint between the source and receiving points a predetermined distance from the respective coordinate point such as to exclude energy which could have been returned by a specular reflector within a predetermined dip range.
 5. The method of claim 4 wherein the coordinate points define three spacial dimensions.
 6. The data image produced in an automatic data processing machine to determine the location of impedance discontinuities in a wave propagating medium from a plurality of digital input records each representative of wave energy returning to a receiving point from wave energy induced at a source point by establishing a plurality of coordinate points defining the data image in at least two spacial dimensions, each coordinate point having a composite value produced by combining a plurality of weighted amplitude values, each weighted amplitude value being derived by picking the amplitude value on a different input trace at the time required for wave energy to travel from the source point of each of the respective input records to the coordinate point and return to the receiving point for the respective input record and multiplying each of the pick amplitude values by a weighting factor that is a function of the depth and the horizontal distance between the source points and receiver points of the input trace from which the amplitude value is picked and the coordinate point for which the value is to be derived, wherein the weighting factor at a given depth increases as the horizontal distance increases to enhance the image of diffractors.
 7. The data image of claim 6 wherein the coordinate points define three spacial dimensions.
 8. The data image produced in an automatic digital data processing machine to determine the location of impedance discontinuities in a wave propagating medium from a plurality of digital input records each representative of wave energy returning to a receiving point from wave energy induced at a source point by establishing a plurality of coordinate points defining the data image in at least two spacial dimensions, each coordinate point having a composite value produced by combining a plurality of weighted amplitude values, each weighted amplitude value being derived by picking the amplitude value on a different input trace at the time required for wave energy to travel from the source point of each of the respective input records to the coordinate point and return to the receiving point for the respective input record and multiplying each of the picked amplituDe values by a weighting factor that is an inverse function of the degree of correspondence between preselected amplitude values, the degree of correspondence being indicative of spectral reflectors.
 9. The data image of claim 8 wherein the coordinate points define three spacial dimensions.
 10. The data image produced in an automatic data processing machine to determine the location of diffractive impedance discontinuities from a plurality of digital input records each representative of wave energy returning to a receiving point from wave energy induced in a wave propagating medium at a source point by establishing a plurality of coordinate points defining the data image in at least two spacial dimensions, each coordinate point being a composite value derived from a combination of amplitude values, each amplitude value being the amplitude value at the time on a different input trace required for energy to travel from the source of the respective input trace to the coordinate point to the receiving point for the respective record, the amplitude values for each coordinate value being selected substantially only from those input traces having the midpoint between the source and receiving points a predetermined distance from the respective coordinate point such as to exclude energy which could have been returned by a specular reflector within a predetermined dip range.
 11. The data image of claim 10 wherein the coordinate points define three spacial dimensions. 